This invention relates to projection transparencies made with hot melt ink and to methods for making such transparencies.
Hot melt inks are used in thermal transfer printers and in certain ink jet printers. The characteristic of these inks is that they are solid at room temperature, liquefied by heating for marking, and resolidified by freezing on the marked substrate.
Transparency substrates are made of transparent sheet material, such as a polyester material, which is usually not receptive to liquid materials such as water- and glycol-based inks. When these solvent-based inks are used to make transparencies, the substrate is coated with a layer receptive to the ink and the ink is absorbed into the coating. For example, U.S. Pat. Nos. 4,528,242 to Burwasser, 4,547,405 to Bedell et al., 4,555,437 to Panck, 4,575,465 and 4,578,285 to Viola, and 4,592,954 to Malhotra disclose special coatings which are capable of absorbing inks for transparent base material such as Mylar. Hot melt inks, however, generally can be formulated to wet and adhere to such substrates, but they do not penetrate into the substrate or into a coating on the substrate. Instead, they adhere to the substrate surface and retain a three-dimensional form. In this way they are distinct from inks which are absorbed or dry into a flat spot through evaporation or absorption. Moreover, transparencies differ from fibrous substrates such as described in Japanese Published Application No. 62-135370 in that spreading of the ink will not improve adhesion by absorption.
A colored hot melt ink image created on the surface of a transparent substrate may be composed of individual drops of the ink as supplied in the ink jet drop-generating process, couples of drops, lines of drops or large areas covered completely by drops. Light passing through the surface of the deposited ink is refracted by the local curvature of the ink surface. A first deficiency in color projection occurs when the curvature is large, i.e., the radius of curvature is small, because the light is deflected through a large angle from its original direction and may be lost from the optical path of the projection apparatus. The projected image of this area of the transparency appears dark. If the radius of curvature of the surface is large, light which passes through the substrate and the ink is refacted only slightly and is collected by the projection lens. Hence it is advantageous if the local radius of curvature of the surface of the ink image is sufficiently large over the entire surface of the image. For individual drops of specified volume, the large radius of curvature corresponds to a small contact angle between the ink surface and the transparency substrate. It has been found to be most difficult to render transparent via geometry individual, nonagglomerating spots, lines being somewhat easier and solid area coverage being the easiest. The reason is that single droplets have the greatest ratio of edges to surface area and these edges have the steepest surface angles. Hence, most of the discussion hereinafter will be in the context of individual spots of ink.
In the case of black-and-white transparencies, the major concern is that the deposited ink be able to block or reduce transmission of light through the transparency. However, for this projection of color images, it is necessary for the ink to absorb selected wavelengths and pass significant fractions of the remaining wavelengths in order to produce an image with the correct colors.
When projected from a transparency, the deposited hot melt three-dimensional colored ink spots tend to project gray or black images because of any of three loss mechanisms, i.e., refractive scattering of transmitted light by the droplet in the manner of a dioptric lenticule, surface losses resulting from microroughness (frosting) on the order of one micron, and bulk losses resulting from the formation of crystals within the droplet which have a different index of refraction than the other material in the droplet. The small lenticules formed by the three-dimensional ink spots refract light which passes through them away from path to the projection lens so that they cast gray shadows in projection irrespective of the color of the ink which forms the lenticule.
For naturally amorphous (noncrystalline) materials, the microroughness (frosting) and bulk losses are small, i.e., the spots are glassy and "clear". Unfortunately, as is known in the art, the organic materials which are amorphous and which may be fluid enough to jet at temperatures of 100.degree. C. to 160.degree. C. tend to be very soft at room temperature. Consequently, the durability of the ink on a transparency may be inadequate. Generally, inks which have adequate hardness and which are jettable at temperatures of 100.degree. C. to 160.degree. C. are usually crystalline to a significant extent. Such high crystallinity produces light transmission losses and causes "opacity" of the ink drop. The bulk losses and surface roughness, i.e., frosting, are also a result of the ordered arrangement of the molecules into a plurality of randomly or obilquely oriented or disoriented crystals. hence crystalline inks tend to have a high degree of surface and bulk scattering, producing light transmission losses greater than 50%, so as to project "gray" spots rather than spots with high color purity. On the other hand, such inks are generally suitable for black-and-white transparencies.
Attempts have been made to overcome such problems by pressing the three-dimensional ink spots on the transparent substrate to flatten them as described, for example, in U.S. Pat. No. 4,745,420, but the flattening affects only the uppermost central portions of the spots, leaving the peripheral portions of the ink spots curved so as to refract most of the light passing through the spots away from the path to the projection lens. Some improvement may be gained by heating the image when pressing it in order to reduce the modulus and yield strength of the ink. Nevertheless, although pressing the three-dimensional ink spots in a transparency to flatten them may produce a slight improvement, the images made in this manner are still unsatisfactory.